Make some sorely needed changes to the SQL schema, and update the code

Make some sorely needed changes to the SQL schema, and update the code
accordingly (I really hope I fixed everything; I've done some testing, and
it seems largely OK). The biggest change is that the event primary key is
now (vhost,event) and not just (event), which is rippling through all the
tables; this means the vhosts are finally truly independent. Also clean up
to remove some old length restrictions from the MySQL days.